The Lewis Concept

The first framework we learned for dealing with acids and bases, the Arrhenius concept, was focused on the question, "What Ions are Produced?" Acids are substances that produce hydrogen ions in solution and bases are substances that produce hydroxide ions. The Brønsted-Lowry concept shifted the question to, "What Species are Donating/Accepting Protons?" Acids are proton donors and bases are proton acceptors. Our final framework, the Lewis Concept, is centered on the details of what is happening with the electrons that make up bonds in acids and bases.

Electron Pair Donors & Acceptors

We have seen in the previous lesson how ammonia can act as a Brønsted-Lowry base because it is a proton acceptor. Let's look at the diagram below. It shows an ammonia molecule accepting a proton to form an ammonium ion. Now let's focus specifically on the electrons that are changing in this process.

Initially, ammonia has three bonds and one lone pair of electrons. It encounters the H⁺ ion, which has no electrons of its own (as mentioned last lesson, this "naked proton" is a problematic idea, but for now it will be useful in illustrating the Lewis concept). In the final product, the two lone pair electrons have been used to form a new bond between N and H.

This illustrates the definition of an acid and base under the Lewis concept: a base is an electron pair donor, and an acid is an electron pair acceptor. Under the Lewis definition, every acid-base reaction involves making a new bond; the base in the reaction is the molecule or ion that provides the electrons for this bond.

It's important to note that the Lewis definition doesn't contradict anything about the Brønsted-Lowry definition, because any time something "accepts H⁺" it must be donating an electron pair, because H⁺ can't provide any electrons to the new bond. Likewise, and "H⁺ donor" is automatically an electron pair acceptor. The Lewis concept of acid/base chemistry is fully inclusive of the Brønsted-Lowry definition.

However, the Lewis concept also broadens significantly what can be considered an acid and a base. So far, all the acids we have encountered contain hydrogen, because there is no other way for a substance to be an "H⁺ donor." However, under the Lewis definition, it is possible to have an acid that doesn't contain any hydrogen atoms. An example is boron trifluoride, shown below reacting with a fluoride ion to form the tetrafluoroborate ion polyatomic ion. As you can see, an electron pair from fluoride is being used to make a new bond to boron. This makes fluoride the Lewis base in the reaction, and BF₃ is the Lewis acid.

We will not spend much time on Lewis acids and bases in this class, but you should be familiar with the definition, and be able to identify the Lewis acid and Lewis base in a reaction where you are shown the molecular structures, like this one.